Radon .

[leswold]

Years ago my folks had this & fitted a small air pump which slightly raised the pressure (in winter at least when the windows were shut) forcing any build up out thru high level vents.

[cornish wood burner]

We put a few detectors around a couple of years ago and found our house was in the slight improvement advised bracket. I drilled 110mm holes through the oversite concrete dug some sumps, linked them together then fed the pipe into a disused chimney. Extra underfloor ventilation would probably have been OK at our level, but as we were fitting a new floor in one room it made the job easy.